CompoundValue.swift

//
//  Copyright © 2017 Tony Arnold. All rights reserved.

enum CompoundValue<Element: Hashable> {
    case distinct(Element)
    case indistinct(Set<Element>)
    case empty

    var distinctValue: Element? {
        switch self {
        case .distinct(let element):
            return element
        default:
            return nil
        }
    }

    var indistinctValues: Set<Element> {
        switch self {
        case .indistinct(let elements):
            return elements
        default:
            return []
        }
    }
}

extension Set {

    var compoundValue: CompoundValue<Element> {
        if count > 1 {
            return .indistinct(self)
        } else if let value = first {
            return .distinct(value)
        } else {
            return .empty
        }
    }
}

extension Collection where Element: Hashable {

    var compoundValue: CompoundValue<Element> {
        return Set(self).compoundValue
    }
}